Underground storage tanks are used to store hazardous substances and petroleum products. It is estimated that a significant proportion of the nearly five million tanks in the United States are leaking harmful products into the environment. To ameliorate this problem, the Environmental Protection Agency (the "EPA") has recently promulgated regulations which require that any leakage exceeding a rate of 0.05 gallons per hour be detected and contained.
Methods for detecting leaks in underground storage tanks are well known in the prior art. Most of these techniques use a quantitative approach to identify a leak or to determine leak rate based on a measurement of volumetric changes of the stored product in the tank. The capability of prior art leak detection methods to accurately measure leakage is affected by certain variables such as temperature change, tank deformation, product evaporation, tank geometry and the characteristics of the stored product. The most significant of these factors is temperature variation, which causes dynamic expansion or contraction of the stored product on both a short-term and long-term basis. Indeed, changes in ambient temperature throughout the day are often large enough so as to "mask" the leakage rate to be measured. For example, a change of 0.01.degree. F. per hour in a 10,000 gallon tank will cause a 0.068 gallon change in the product volume per hour, thus offsetting or amplifying an observed leak rate.
Most of the prior art methods for leak detection attempt to compensate for such temperature variations. In quantitative techniques, i.e., tests based on product volume changes, temperature in the tank is typically sensed by a plurality of temperature sensors located at various levels or stratifications therein. The sensed temperature data is collected and processed to measure the volumetric average of product temperature during a test. Other techniques attempt to compensate for temperature variations by performing the leak test over very short or long time intervals. All such techniques are unsuccessful because of the difference between the measured temperature change and the actual temperature change during the applicable test interval.
One potential solution to the problem of temperature-induced volumetric changes in underground storage tank leak detection involves the use of a two-tube laser interferometer system. This system consists of two laser interferometers attached respectively to two equal length tubes extending to the bottom of the tank. Each tube contains an aluminum float having a corner cube reflector. A signal processing circuit is used to process the data. The technique simultaneously measures the difference in the height of the product in each tube, with one of the tubes being open and the other closed but both tubes initially filled to the same level. Height changes in the open tube caused by thermally-induced volume changes are compensated for by subtracting the height changes in the closed tube. Although this system successfully reduces the effects of temperature-induced volume changes during leak detection, it is extremely costly to implement and operate. Such techniques are thus wholly impractical for widespread commercial use.
Accordingly, there is a need for a reliable and economical method and apparatus for eliminating temperature effects in a storage tank leak detection system which overcomes these and other problems associated with prior art techiques.